1. Field of the Invention
The invention relates to an improved microwave system for measuring the moisture content of soil.
2. Description of the Prior Art
Evaluation of soil water content is a fundamental operation for irrigation scheduling in crop production, management and research. To date, the most effective method of measuring the soil water in the root zone is by the TDR technique. Numerous researchers over the last 20 years have shown the TDR technique provides a reliable measure of soil moisture through the estimation of the soil's dielectric constant, Heathman G. C.*, Starks P. J and Brown M. A. 2003. Other inventions based upon the TDR technique include U.S. Pat. Nos. 6,657,443, 5,726,578, 4,918,375.
In traditional TDR measurements, a rapid rise-time short duration, ns-pulse is transmitted down a transmission line to an open-ended buried waveguide sensing-structure where the signal undergoes a reflection back towards the signal source due to the mis-matched impedance of the end of the waveguide, i.e., the end of the buried structure. As the signal spends a portion of it's time traveling down the buried waveguide sensing-structure, the transit time of the pulse is delayed in proportion to the dielectric constant of the material surrounding this buried waveguide sensing-structure. The measurement consists of quantifying this variable delay to provide an estimation of the surrounding material's dielectric constant. As the TDR technique utilizes a very fast pulse that travels down the transmission line at a velocity equal to approximately 1/10 to ½ times the speed of light, in order to provide a reasonable dynamic range for the measurement, an extremely fast clock must be provided that operates at 100 to 1000 times this speed. Thus, the TDR technique while accurate, requires very expensive circuitry in order to accurately measure the delay (GHz or faster clock). This has lead to the TDR technique being predominantly used by only the research community at the cost of neglecting the bulk of the irrigation market that is comprised of growers, and landscapers. Given the difficulties in creating circuits to perform this measurement, even today and since 1980, the TEKTRONIX cable tester, model 1502, has been the instrument of choice for researchers seeking to obtain TDR or reflectometric measurements of soil moisture. This test instrument operates with a step pulse rise-time (the transition time a square wave takes to change the voltage from it's low level to it's maximum level) having 145 ps rise time (G. C. Topp and J. L. Davis, Measurement of Soil Water Content using Time-domain Reflectometry (TDR): A Field Evaluation. Soil Sci. Soc. Am. J., vol. 49, 1985,: 19–24). In an attempt to reduce the cost of TDR, several inventions have been developed that seek to slow down the pulse, such as U.S. Pat. No. 5,818,214, it should however be noted that this invention has only marginally reduced the demands of the requisite circuitry as it is still dependant upon the transmission of a marginally slower pulse.
Another disadvantage to the TDR method is due to the very broad frequency band-width the system requires due to the technique's use of a very rapid rise-time ns-duration pulse. This disadvantage is best illustrated by noting that the frequency spectrum of a pulse is extremely wide and when taken to the extreme with an infinitely narrow pulse-width, leads to an infinitely wide frequency spectrum. Thus, this technique is, by it's design, an extremely broad-band technique that cannot take advantage of the frequency-based variability that naturally occurs in the dielectric spectrum. Thus, the response of TDR is limited to an average response of all of these frequencies that produce a combined response that becomes the TDR measurement. Alternatively, a frequency based measurement that utilizes a small portion of a narrow bandwidth provides an additional ability to obtain more precise information as it's free to chose the band of interest to characterize the required trait of interest that is unavailable to the TDR user. As an example application where a frequency based measurement can out-perform TDR; it is well known that salinity provides a very strong response at low frequencies, kHz to low MHz, while conversely, at microwave frequencies the salinity response is very small in comparison to the strong signature provided by the water. Thus, the ability to tune the frequency to bands of specific absorbance allows for accurate independent characterization of more than one species such as measurement of both salinity as well as the soil water content, or the measurement of soil-water independently from the salinity levels present in the soil.
In an effort to reduce costs, other patents have taught alternative methods to TDR that seek to provide lower cost methods of measuring the dielectric constant of the soil; such as U.S. Pat. No. 5,148,125. The technique illustrated in U.S. Pat. No. 5,148,125 utilizes a buried loop transmission line coupled to a resonant circuit. Unfortunately, however resonant structures typically are extremely temperature sensitive and as such have had very limited success in the industry as well as the research community due to the difficulties in calibrating these types of instruments across the temperature range of interest and use. Other sensors have utilized resistance and capacitance probes in either timing or resonant circuits such as U.S. Pat. No. 5,341,673. The main trouble with these types of instruments lies in their inability to work at microwave frequencies which is where the effect of salinity no longer affects the measurement. Thus, these lower frequency systems are subjected to a dependence upon the soil's salinity as a confounding element affecting the reading.
U.S. Pat. No. 2,659,860 teaches a method to measure the moisture content of bales of material, by directing a 10 GHz microwave beam through the bale and receiving the beam with another antenna on the far side of the bale from the one which generated the signal. The moisture content of the bale is then determined solely from the attenuation of this signal.
Meyer and Schilz U.S. Pat. No. 4,361,801 teaches a sensing technique that requires measurements of both attenuation and the phase delay of propagation in order to calculate the real and the imaginary components of the complex permittivity measurement in order to measure moisture at 9 GHz which is independent of density. The basis for this measurement is the ratio of the complex permittivities providing, which is a modification of taking the ratio of the attenuation to the propagation delay, as the measure of moisture (either as phase delay or equivalently the time delay). Nelson et al. U.S. Pat. No. 6,147,503 describes another moisture sensor algorithm that provides a moisture sensor that is independent of density over the narrow range of densities provided by loose seed kernel samples versus tightly packed seed kernel samples. They teach a technique that operates at 11.3 and 18 GHz again using both the attenuation and the propagation delay to calculate the complex permittivity of the material to derive an algorithm for the determination of the moisture content of the material. Moshe et al. U.S. Pat. No. 6,476,619 describes a microwave cavity perturbation technique for the sensing of moisture and or density in cotton sliver that has a preferred operating range of 7–9 GHz. In the perturbation technique the system is setup with a resonant peak in the signal amplitude versus frequency plot and utilizes the frequency change in the location of this peak as the measure of permittivity change thereby providing a measure of the permittivity from which the moisture content can be estimated assuming a constant density of material. Moshe et al. U.S. Pat. No. 6,111,415 describes the use of the well known radar technique of Frequency Modulated Time Domain “FMTD” for use as a density sensor which is used to correct an attenuation based moisture sensor. Other patents by Moshe et al. include U.S. Pat. Nos. 5,845,529 and 6,107,809 which utilize a ratio of attenuation to phase delay measurement in a manner very similar to the Meyer and Schilz U.S. Pat. No. 4,361,801. The reoccurring theme between all of these patents is that they all use very high microwave frequencies, typically above 7 GHz, and all of them utilize a measure of the attenuation of the signal after it has been transmitted through the material under test as the primary measure of the moisture content. As such, all of these patents provide very expensive solutions.
However, despite these improvements, the need remains for a low cost, accurate technique for the determination of soil moisture content.